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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

MILES AND GRESTNELL GLACISES                            425
represents an area in which the ice has been retreating rapidly in the period since before 1888, presumably because the valley glacier is not moving forward strongly, is not protected at the terminus by morainic debris, and is undercut by the water. The site of the lake may possibly be classed as analogous to the interior flat areas observed in other bulb glaciers.
In 1910, the lake was from one to two miles wide, Sf miles long, and 150 to 800 feet deep (PI. CLXV). Since the low-water surface is 116 feet above tidewater, the bottom, therefore, descends to 184 feet below sea level (Fig. 64). So large a lake in the course of a heavily-loaded river, and particularly of a glacial stream like Copper River, is an unusual feature, for such sediment-laden rivers rapidly fill and destroy lakes. The presence of the lake is of itself clear evidence that the glacier has recently receded from the site of the lake basin. The great depth of the lake is evidence along the same line, for at the mouth of Copper River the deposits have built up a large delta extending out into the Pacific Ocean, and it is necessary to go over 20 miles offshore before a point in the ocean is reached with a depth as great as that of Miles Lake.
Southern Terminal Moraine. South of the lake is a strip of valley bottom a quarter to a hah* mile wide which also represents parts of the former bulb of Miles Glacier.
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FIG. 64.   NATURAL SCALE LONGITUDINAL SECTION OF MILES GLACZHB INT 1910.
This is proved by the crescentic terminal moraine which swings across the Copper River valley from the eastern valley wall a half mile south of Miles Glacier to the railway bridge at Miles Glacier station (PI. CLXV). This moraine is 70 to 90 feet higher than the outwash plain on the south and 0 to 30 feet higher than the area of ground moraine which separates it from the lake. One kame hill in this moraine rises to a height of 186 feet above the low-water surface of the lake.
Most of the moraine is a series of discontinuous ridges forming a belt varying in width from 100 feet to a quarter mile, and with a general trend parallel to the former ice front. In the kettles between these ridges are a number of ponds.
Ground Moraine and Detached Ice Mass. Between the terminal moraine and the lake is a parallel strip of ground moraine and one large, detached ice mass. The ground moraine is very irregular and has many deep kettles containing pools, and a rolling surface of low ridges, standing 60 to 100 feet higher than the outwash plain south of the moraine. It is made up of till and gravel, and, in large areas, of angular rocks of considerable size (PI. CLXVI), sometimes arranged in long lines of crevasse deposits.
The edge of the detached ice mass south of Miles Glacier is buried beneath the out-wash gravels which mantle part of the ground moraine, and these gravels were slumping in 1910. The detached ice mass is riven by great crevasses, and one of these, 50 to 90 feet deep, had morainic debris sliding down into it (PL CLX VT[ , A) from the thick covering of ablation moraine resting on the ice. When the ice h-as completely